NEW YORK -- Former NBA referee Tim Donaghy, who's being investigated by the FBI for betting on games he worked, had law enforcement officials at his Florida home after receiving two threatening phone calls.

Both calls seemed to come from the same unidentified caller, Manatee County Sheriff's Office spokesman Randy Warren said.

``There is reason for us to keep an eye on his place and follow up,'' he said.

Three squad cars arrived at Donaghy's home on Sunday after he received the calls, according to a sheriff's office report. Donaghy did not have a listed phone number at his Bradenton home.

A person with knowledge of the FBI investigation said the NBA was unaware of the FBI investigation until after the NBA finals. The person requested anonymity because he was not authorized to discuss the ongoing case.

According to a law enforcement official, authorities are examining whether the referee made calls to affect the point spread in games on which he or associates had wagered thousands of dollars over the past two seasons.

The official spoke on condition of anonymity, because he wasn't authorized to discuss the ongoing investigation.

The referee had a gambling problem and was approached by low-level mob associates through an acquaintance, the official said.

The New York Daily News, citing unidentified law enforcement sources, reported on Sunday that Donaghy will cooperate with authorities and possibly name other officials and players involved in the betting scandal.

On Monday, the newspaper reported former neighbors of Donaghy in Pennsylvania said they were approached more than a year ago by a private investigator they believed was hired by the NBA to check into Donaghy's gambling habits.

Kit Antsey, a real estate agent in West Chester, Pennsylvania who helped Donaghy buy a home, told The Daily News a private investigator contacted him 18 months ago and asked him whether Donaghy bet on sports and at an Atlantic City casino.

NBA commissioner David Stern held a press conference Tuesday, discussing the investigation.